What software are you using? MacTheRipper is what I've heard is good, there's also OSEx. Someone else can tell you more. I never use my Mac for editing. Guide for ripping with OSEx and if you go up, for using iMovie for makign AMV's Here

When you are done with your video in iMovie you will want to export the video from iMovie. With that export you will want to use ffmpegX to compress it to XviD.

OSex is a good DVD ripper, but it is no longer being developed. So MacTheRipper would be the better choice since it is still be updated, and ther are so many more option you can do with it. Here is the link for you.

Sorry for the double post, but the MacTheRipper link I gave you was wrong. If you clicked on it, it takes you to a page that tells you it has been shut down. Well that really is not true.
Here is a link that will let you download it.
Here is the MacTheRipper page.